Numerical and experimental evaluation of continuous ultrasonic sludge treatment system

Ultrasonic disintegration is a very promising sludge pretreatment method that leverages the cavitation effect to produce extreme physical environments characterized by high temperatures and high pressures. This process disintegrates sludge structure features, promotes sludge dewatering, and aides resource recovery. This paper presents a newly designed continuous ultrasonic sludge treatment device. The characteristics of the ultrasonic wave propagated in the activated sludge were simulated, with the results showing that at lower frequencies, the acoustic pressure energy distribution exhibits more local concentrations, whereas at 80kHz, the energy distribution is relatively uniform as a result of the interference of standing waves. Subsequently, activated sludge was ultrasonically treated with different exposure times and frequencies. The sludge's capillary suction time, particle size, and moisture content were measured. The results showed different trends for each of the investigated parameters. The dewatering performance was best when the exposure time was 5-10s. Finally, different substances were added to the ultrasonically treated sludge to analyze the effects of ultrasonic treatment on anaerobic digestion. The gas production rate was higher when glucose was the added substance than it was for yeast. The highest total concentration of produced gas, including both hydrogen and methane, was 34% for an ultrasonic input power of 200W at a 25kHz frequency, an exposure time of 20s, and with 30g of added glucose. The gas production rate was found to be higher at the lower frequency when frequency was the only variable. These experiments demonstrate that ultrasonic treatment can change the structure of sludge particles and the moisture content of the sludge, improving sludge dewatering performance. Furthermore, after ultrasonic treatment can improve gas production.